A piezoelectric material can make mechanical-to-electrical energy conversion or electrical-to-mechanical energy conversion. Examples of the typical piezoelectric material include lead zirconate titanate (Pb(Zr, Ti)O3) (also referred to as “PZT” in the following). Particularly, PZT having a perovskite-type tetragonal crystal structure can achieve the largest piezoelectric displacement in a c-axis (<001> axis) direction. However, most piezoelectric materials are a polycrystalline material composed of aggregates of crystal grains whose crystal axes are oriented variously. Therefore, the spontaneous polarization Ps is arranged variously as well.
With the recent progress in miniaturization of electronic equipment, there has been a strong demand for a smaller piezoelectric element. To meet the demand, the piezoelectric element is being formed as a thin film having a much smaller volume than that of a widely used sintered body. Thus, the research and development on the piezoelectric element in the form of a thin film have been conducted actively. For example, PZT has the spontaneous polarization Ps that orients in the c-axis direction. Therefore, it is necessary to align the c-axis of crystals that constitute a PZT thin film in a direction perpendicular to the substrate surface so that the PZT thin film can have high piezoelectric characteristics. To achieve this, a conventional technique uses a magnesium oxide (MgO) single-crystal substrate that has a rock-salt crystal structure and is cut so that a plane of crystal orientation (100) is exposed to the surface, and a PZT thin film with the c-axis (<001> axis) oriented perpendicularly to the surface. The PZT thin film is formed on the MgO single-crystal substrate at 600° C. to 700° C. (e.g., J. Appl. Phys. vol. 65, No. 4 (15 February 1989) pp. 1666-1670 and JP 10(1998)-209517 A).
However, the thermal expansion coefficient of the MgO single-crystal substrate is 120×10−7/deg, and that of PZT is 56×10−7/deg. When the PZT thin film is formed on the MgO single-crystal substrate, the PZT thin film is subjected to a large compressive stress due to a difference in thermal expansion coefficient while it is cooled from the deposition temperature (600° C. to 700° C.) to room temperature. Therefore, the entire PZT thin film shrinks considerably, making it impossible to provide the property of extending and contracting in accordance with electrical energy, i.e., high piezoelectric characteristics. Even if the MgO single-crystal substrate is removed by etching, the compressive stress remains in the PZT thin film and cannot be relaxed completely because each ion in the crystal does not diffuse at about room temperature. Thus, a large piezoelectric displacement cannot be achieved.